WSRequest.post(Document) set incorrect encoding in the XML Prolog

I use JAXB to marshal a Java Object to Document, then use WSRequest.post() to send the documet to a server. When marshaling, I set the encoding to EUC_KR:
...
Document document = db.newDocument();
marshaller.setProperty(Marshaller.JAXB_ENCODING, "EUC_KR");
marshaller.marshal(obj, document);

The POST request is:
ws.url(registerProductUrl)
.addHeader("Content-Type", "text/xml;charset=euc-kr")
.post(document);

This works well in Play 2.5.14, where the prolog in the request is
<?xml version="1.0" encoding="EUC-KR"?>

When upgrade to play 2.6.0, the prolog in the request changed to:
<?xml version="1.0" encoding="UTF-8"?>
that is, the encoding is now not "EUC-KR.

I do not find where WSRequest.post(Document) is implemented, but I find that play.libs.ws.XML.toBytes(Document document) has bug. It does not set the encoding when transform a XML document.

The following project can produce the issue:
https://github.com/guofengzh/play-xml-prolog-issue

see JaxbText.java in test/controller.
JaxbText.XMLtoBytesTest() use XML.toBytes() to reproduce the issue. The prolog it generated is:
<?xml version="1.0" encoding="UTF-8"?> (the correct encoding should be "EUC-KR".

JaxbText.myToBytesTest() use the modified toBytes(), which set the encoding using:
transformer.setOutputProperty(OutputKeys.ENCODING, encoding);
and generate the correct encoding in the prolog:
<?xml version="1.0" encoding="EUC-KR"?>

Start with play.libs.ws.XML.toBytes(Document) and compare its behavior with JaxbText.java in test/controller. Run JaxbText.XMLtoBytesTest() and compare it with JaxbText.myToBytesTest(); the issue is done when WSRequest.post(Document) preserves the document's EUC-KR encoding in the XML prolog.